The invention is based on a priority application EP 00440237.6, which is hereby incorporated by reference.
The invention relates to a method of extending the capture range of a wavelength monitor for the lasers of a wavelength division multiplex (WDM) transmission system wherein the capture range comprises one wavelength period of a periodic error signal generated with the aid of the wavelength filter, the capture range contains a desired wavelength of a plurality of equidistant wavelengths, each of the lasers of the WDM transmission system is set at a desired wavelength (λ0) by comparing the error signal (E) with a comparison value (C1 or C2), that is unique in the capture range (CR) for a chosen slope sign, the wavelength period of the error signal is set such that it corresponds to double the wavelength spacing of two adjacent wavelengths of the WDM transmission system and the desired wavelength is set taking into account the slope sign of the error signal, to a wavelength monitor with a wavelength filter and means of generating a periodic error signal for setting a laser, where the capture range of the irradiated wavelength comprises one wavelength period of the error signal and contains a desired wavelength of a plurality of equidistant wavelengths of a WDM transmission system, and to a laser system with lasers and at least one wavelength monitor, wherein each of the lasers is set at a desired wavelength with the aid of an error signal.
WDM methods are increasingly being used in optical transmission systems. In such methods a number of modulated optical carriers with differing frequencies are transmitted simultaneously in a glass fibre. Each of these carriers forms a channel which is logically independent of the carriers, each channel being fed by one laser. To increase the transmission capacity, the number of channels of WDM transmission systems is increasingly being enlarged, the frequency spacing and thus also the wavelength spacing being increasingly reduced. In present-day transmission systems with so-called dense WDM (DWDM), referred to in the following as DWDM transmission systems, for example 16 channels are transmitted with an equidistant frequency spacing of 100 GHz. To further increase the transmission capacity, in accordance with the International Telecommunication Union (ITU) it is proposed that this frequency spacing be halved to 50 GHz. However, with decreasing frequency spacing, the demands on the accuracy and stability of the wavelengths emitted by the laser diodes, also referred to in the following as ITU wavelengths, become greater. The ITU permits a wavelength deviation of a maximum 10% of the wavelength spacing between two adjacent channels, also referred to in the following as ITU wavelength spacing.
The wavelength of a laser diode, abbreviated to laser in the following, is a function of its temperature. With the aid of a controllable laser heating unit, this temperature is set such that the desired wavelength is emitted. For this purpose, with the aid of a wavelength monitor, a wavelength-dependent error signal is generated, from which a suitable control signal is formed for controlling the laser heating unit. In the wavelength monitor, the fed-in laser light is split by a splitter in two optical branches; in the first optical branch the light is directly fed to a first photodiode, while the light in the second optical branch passes through a wavelength filter before striking a second photodiode. While the first photodiode of the wavelength monitor supplies a current which is proportional to the intensity of the fed-in laser light independently of the wavelength, the second photodiode supplies a current periodic with the wavelength. The error signal is generated for example by forming the difference between the output currents of the photodiodes. In order that all the lasers of a DWDM transmission system can each be set at a ITU wavelength with the aid of a wavelength monitor, the period spacing, also known as the free spectral range (FSR), of the wavelength filter must correspond exactly to the ITU wavelength spacing. To achieve in each case an unequivocal setting of the lasers at a specific ITU wavelength, it must be ensured that, before the wavelength control comes into effect, the emission of each laser always falls within a specific wavelength range. This wavelength range, also referred to in the following as capture range (CR), corresponds to the free spectral range, whereby a capture range in each case contains only the desired ITU wavelength.
As a result of the previously described, planned reduction in the ITU wavelength spacing, the free spectral range of the wavelength filter is reduced proportionally. Consequently the capture range is also reduced proportionally.
Lasers undergo a shift in the emitted wavelength due to ageing. The capture range should therefore be sufficiently large to ensure that the emitted wavelength still falls in the capture range even after ageing. If the capture range is too small, the risk exists that after a certain period of time the wavelength will be outside the capture range and therefore can no longer be set at the provided ITU wavelength.
When a Fabry Perot (FP) interferometer is used as wavelength filter, also referred to in abbreviated form as FP interferometers in the following, the reduction in the ITU wavelength spacing also leads to an enlargement of the air gap, which runs counter to the endeavoured increasing integration of the optical components.